1. Field of the Invention(s)
The present invention(s) relate to local oscillator leakage, and more particularly, to cancelling local oscillator leakage at a transmitter by way of information provided by a receiver.
2. Description of Related Art
Single carrier frequency domain equalization (SC-FDE) is one of many approaches used for facilitating wireless communication. An advantage of SC-FDE over single carrier systems is computational efficiency of channel equalization since equalization is performed in frequency domain. In the frequency domain, channel equalization is a product of frequency transforms of a signal and an inverse of channel estimation, while in single carrier systems, channel equalization is a convolution of a signal and equalizer impulse response. An advantage of a SC-FDE over multi-carrier systems is lower peak to average power ratio (PAPR). SC-FDE systems operate in frames, with each frame starting with a preamble used for channel estimation and with each preamble being followed by data blocks (or blocks).
A disadvantage of SC-FDE systems is that signals are sent, received, and processed in blocks, rather than in samples. Block processing limits Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) algorithms, and often causes delays among components of a communications receiver. Additionally, certain algorithms of a communications receiver still require sample-based processing and must be performed in the time domain, such as carrier recovery using a carrier phase-lock loop (PLL). Carrier recovery PLL is required for correcting carrier frequency offset, correcting phase offset, and filtering out carrier phase noise. Since a carrier recovery PLL is positioned after frequency-domain processing in SC-FDE systems (e.g., after FFT, channel estimation, equalization, and DC offset measurement), frequency-domain algorithms in such systems are affected by the carrier frequency and phase rotation. In addition, since blocks in SC-FDE systems are separated by a cyclic prefix (CP) when transmitted over the air-link, the CP removal in the communications receiver causes discrete jumps in carrier phase from block to block, prior to carrier recovery.
A disadvantage of direct conversion transceiver architectures is the local oscillator (LO) leakage generated in a transmitting device's IQ modulator due to the modulator imperfections. This LO leakage is converted on the receiving device as a direct current (DC) offset signal in I and Q baseband signals. The DC offset signal affects the received quadrature amplitude modulation (QAM) symbol constellation by shifting the constellation away from the Gaussian plane origin and, hence, limiting the signal-to-noise ratio (SNR) of the signal received by the receiving device. The situation is exacerbated by carrier frequency offset and phase noise impairments in the signal received, which can change the DC offset signal into a jittery low-frequency signal that usually cannot be removed by the receiving device.